=135. General Principles.=—In the preceding pages the principles
of the volumetric methods of sugar analysis by means of alkaline
copper solution have been set forth. They depend either on the total
decomposition of the copper solution employed by the reducing sugar,
or else on the collection and titration of the cuprous oxid formed in
the reaction. In the gravimetric methods the general principle of the
process rests upon the collection of the cuprous oxid formed and its
reduction to metallic copper, the weight of which serves as a starting
point in the calculations of the weight of reducing sugar, which has
been oxidized in the solution.
The factors which affect the weight of copper obtained are essentially
those which influence the results in the volumetric method. The
composition of the copper solution, the temperature at which the
reduction is accomplished, the time of heating, the strength of the
sugar solution and the details of the manipulation, all affect more or
less the quantity of copper obtained. As in the volumetric method also,
the kind of reducing sugar must be taken in consideration, dextrose,
levulose, invert sugar, maltose and other sugars having each a definite
factor for reduction in given conditions. It follows, therefore, that
only those results are of value which are obtained under definite
conditions, rigidly controlled.
=136. Gravimetric Methods of the Department of Agriculture
Laboratory.=—The process used in this laboratory is based essentially
on the methods of Maercker, Behrend, Morgen, Meissl, Hiller and
Allihn.[105] Where dextrose alone is present, the table of factors
proposed by Allihn is used and also the copper solution corresponding
thereto.
For pure invert sugar, the tables and solutions of Meissl are used. For
invert sugar in the presence of sucrose, the table and process proposed
by Hiller are used.
[Illustration: FIGURE 43. APPARATUS FOR THE ELECTROLYTIC DEPOSITION OF
COPPER.]
The reduction of the copper solution and the electrolytic deposition of
the copper are accomplished as follows:
The copper and alkali solutions are kept in separate bottles. After
mixing the equivalent volume of the two solutions in a beaker, heat is
applied and the mixture boiled. To the boiling liquid the proper volume
of the cold sugar solution is added. This must always be less than the
amount required for complete reduction. The solution is again brought
into ebullition and kept boiling exactly two minutes. A two-minute
sand glass is conveniently used to determine the time of boiling. At
the end of this time an equal volume of freshly boiled cold water is
added, and the supernatant liquor at once passed through a gooch under
pressure. The residual cuprous oxid is covered with boiling water and
washed by decantation until the wash water is no longer alkaline. It
is more convenient to wash in such a way that, at the end, the greater
part of the cuprous oxid is in the gooch. The felt and cuprous oxid are
then returned to the beaker in which the reduction is made. The gooch
is moistened with nitric acid to dissolve any adhering oxid and then
is washed into the beaker. Enough nitric acid is added to bring all
the oxid into solution, an excess being avoided, and a small amount
of water added. The mixture is again passed under pressure through
a gooch having a thin felt, to remove the asbestos and the filtrate
collected in a flask of about 150 cubic centimeters capacity. The
washing is continued until the gooch is free of copper, when the volume
of the filtrate should be about 100 cubic centimeters. The liquid is
transferred to a platinum dish holding about 175 cubic centimeters and
the flask washed with about twenty-five cubic centimeters of water.
From three to five cubic centimeters of strong sulfuric acid are added
and the copper deposited by an electric current.
=137. Precipitating the Copper.=—When no more nitric acid is used than
indicated in the previous paragraph, it will not be necessary to remove
it by evaporation. The platinum dishes containing the solutions of the
cuprous oxid are arranged as shown in the figure for the precipitation
of the copper by the electric current. Each of the supporting stands
has its base covered with sheet-copper, on which the platinum dishes
rest. The uprights are made of heavy glass rods and carry the supports
for the platinum cylinders which dip into the copper solutions. The
current used is from the city service and is brought in through the
lamp shown at the right of the figure. This current has a voltage of
about 120. After passing the lamp it is conducted through the regulator
shown at the right, a glass tube closed below by a stopper carrying a
piece of platinum foil, and above by one holding a glass tube, in the
lower end of which is sealed a piece of sheet platinum connected,
through the glass tube, with the lamp. The regulating tube contains
dilute sulfuric acid. The strength of current desired is secured by
adjusting the movable pole. A battery of this kind easily secures the
precipitation of sixteen samples at once, but only twelve are shown
in the figure. The practice here is to start the operation at the
time of leaving the laboratory in the afternoon. The next morning
the deposition of the copper will be found complete. The wiring of
the apparatus is shown in the figure. The wire from the regulator is
connected with the base of the first stand, and thence passes through
the horizontal support to the base of the second, and so on. The return
to the lamp is accomplished by means of the upper wire. This plan of
arranging the apparatus has been used for two years, and with perfect
satisfaction.
Where a street current is not available, the following directions
may be followed: Use four gravity cells, such as are employed in
telegraphic work, for generating the current. This will be strong
enough for one sample and by working longer for two. Connect the
platinum dish with the zinc pole of the battery. The current is allowed
to pass until all the copper is deposited. Where a larger number of
samples is to be treated at once, the size of the battery must be
correspondingly increased.
=138. Method Used at the Halle Station.=—The method used at the Halle
station is the same as that originally described by Maercker for
dextrose.[106] The copper solution employed is the same as in the allihn
method, _viz._, 34.64 grams of copper sulfate in 500 cubic centimeters,
and 173 grams of rochelle salt and 125 grams of potassium hydroxid in
the same quantity of water. In a porcelain dish are placed thirty cubic
centimeters of copper solution and an equal quantity of the alkali,
sixty cubic centimeters of water added and the mixture boiled. To the
solution, in lively ebullition, are added twenty-five cubic centimeters
of the dextrose solution to be examined which must not contain more
than one per cent of sugar. The mixture is again boiled and the
separated cuprous oxid immediately poured into the filter and washed
with hot water, until the disappearance of an alkaline reaction. For
filtering, a glass tube is employed, provided with a platinum disk,
and resembling in every respect similar tubes used for the extraction
of substances with ether and alcohol. The arrangement of the filtering
apparatus is shown in Fig. 44. In the Halle method it is recommended
that the tubes be prepared by introducing a platinum cone in place of
the platinum disk and filling it with asbestos felt, pressing the felt
tightly against the sides of the glass tube and making the asbestos
fully one centimeter in thickness. This is a much less convenient
method of working than the one described above. After filtration and
washing, the cuprous oxid is washed with ether and alcohol and dried
for an hour at 110°, and finally reduced to metallic copper in a stream
of pure dry hydrogen, heat being applied by means of a small flame. The
apparatus for the reduction of the cuprous oxid is shown in Fig. 45.
The metallic copper, after cooling and weighing, is dissolved in nitric
acid, the tube washed with water, ether and alcohol, and again dried,
when it is ready for use a second time. The percentage of dextrose is
calculated from the milligrams of copper found by Allihn’s table.
[Illustration: FIGURE 44. APPARATUS FOR FILTERING COPPER SUBOXID.]
[Illustration: FIGURE 45. APPARATUS FOR REDUCING COPPER SUBOXID.]
=139. Tables for Use in the Gravimetric Determination of Reducing
Sugars.=—The value of a table for computing the percentage of a
reducing sugar present in a solution, is based on the accuracy with
which the directions for the determination are followed. The solution
must be of the proper strength and made in the way directed. The degree
of dilution prescribed must be scrupulously preserved and the methods
of boiling during reduction and washing the reduced copper, followed.
The quantity of copper obtained by the use of different alkaline copper
solutions and of sugar solutions of a strength different from that
allowed by the fixed limits, is not a safe factor for computation.
It must be understood, therefore, that in the use of the tables the
directions which are given are to be followed in every particular.
=140. Allihn’s Gravimetric Method for the Determination of
Dextrose.=—_Reagents_:
I. 34.639 grams of CuSO₄.5H₂O, dissolved in water and diluted to
half a liter:
II. 173 grams of rochelle salts } dissolved in water and diluted
125 grams of KOH, } to half a liter.
_Manipulation_: Place thirty cubic centimeters of the copper solution
(I), thirty cubic centimeters of the alkaline tartrate solution (II),
and sixty cubic centimeters of water in a beaker and heat to boiling.
Add twenty-five cubic centimeters of the solution of the material to
be examined, which must be so prepared as not to contain more than one
per cent of dextrose, and boil for two minutes. Filter immediately
after adding an equal volume of recently boiled cold water and obtain
the weight of copper by one of the gravimetric methods given. The
corresponding weight of dextrose is found by the following table: